Diastolic dysfunction is the most common cause of congestive heart failure in elderly populations. One year mortality rates for afflicted individuals aged 70 years or older approximate 50%. Therapy options remain empirical. Although very little is known about the etiology of Diastolic Heart Failure (DHF) most studies agree that affected patients exhibit increased ventricular stiffness (reduced chamber compliance). The hypothesis underlying this work is that DHF in elderly populations reflects cross-bridge activity that persists inappropriately during the diastolic (or 'relaxed') phase of the cardiac cycle. These cross-bridges produce an 'active' component of myocardial stiffness that augments the heart's basal (passive) stiffness and impairs ventricular filling by increasing the resistance to inflowing blood. The proposed research will utilize Fischer 344 rats that exhibit aging-associated DHF at 25 months of age. Specific Aim 1 will establish the effects of aging on rat myocardial stiffness. Experiments will test the hypothesis that active stiffness due to inappropriately bound cross-bridges increases to a greater extent with aging than stiffness due to structural components. Intact trabeculae will be isolated from young (5 month) and old (25 month) rats and stretched in the presence and absence of BDM, a cross-bridge inhibitor, to establish the extent of age-dependent changes in active and passive stiffness. Specific Aim 2 will evaluate the effects of altered metabolite concentrations on active stiffness in young and old hearts. Experiments will utilize chemically permeabilized preparations isolated from 5 month and 25 month rat hearts and active stiffness will be assessed by measuring the tension responses to small stretches imposed under sarcomere length control. It is hypothesized that active stiffness due to persistent cross-bridge activity will be enhanced to a greater extent in the old hearts than in the young hearts when the concentrations of hydrogen ions, phosphate ions and ADP are raised to levels mimicking ischemic myocardium. This research is relevant to public health because it investigates the novel hypothesis that aging-related DHF reflects inappropriate contractile activity during the relaxed phase of the cardiac cycle. The experimental results will provide new information about the underlying causes of DHF and should help scientists develop better treatments for diastolic dysfunction in elderly populations. [unreadable] [unreadable] [unreadable]